Alloy steel for cast parts resistant to high temperatures and corrosion



United States Patent 3,165,401 ALLOY STEEL FQR CAST PARTS RESISTANT TOHIGH TEMPERATURES AND CORROSIGN Emil Reusch, Fort Wayne, Ind., assignorto International Harvester Company, Chicago, 111., a corporation of NewJersey No Drawing. Filed Mar. 20, 1957, Ser. No. 647,216

Claims. (Cl. 75-128) This invention relates to a new and improvedstainless steel, and more particularly to a steel suitable for thecasting of internal combustion engine parts, valves and the like, gasturbine parts, etc., which must possess qualities of high strength, highhardness, and high resistance to corrosion oxidation and scaling atelevated temperatures. In the effort of increasing the output ofinternal combustion engines the trend has constantly been to higheroperating temperatures. To avoid certain undesirable results of suchincreased temperatures it has been necessary to add to the fuel,anti-detonants which, however, greatly increase the corrosive effects onparts which come into contact with the products of combustion. In theconventional internal combustion engine, one of these parts which isseriously affected is the engine exhaust valve. The materials heretoforeused have proven to be unsatisfactory in meeting the aforementionedconditions prevalent in present day higher efliciency engines. Exhaustvalves made from conventional materials, forged or extruded, usuallystretched over their whole length, cupped in. the head portion andcracked at the stem close to the head of the valve, which failures inmany instances occurred as a direct result of the high temperatures andthe lack of the material to withstand the effect of the forces acting onthe valve. The increase of the corrosive eifect showed up mainly ineating away the material in the neck portion and on the outer rim of thevalve. It is a fact, therefore, that a good valve material that willmeet present dayrequirements must possess the following characteristics:high hot hardness, high hot strength, good wear resistance, high creepresistance, low thermal expansion; high shock resistance, low ductility,high toughness, high oxidation and corrosion resistance, high thermalconduc tivity and good scaling resistance.

Valves heretofore made by the process of forging or ex truding (known aswrought valves) have been observed to more readily stretch, duringoperation, to the breaking point or generally become quickly brittleresulting in premature failures due to cracking or distorting of the rimof the valve head. In addition to the inadequacies of the materialutilized it is also apparent that such failures are further abetted bythe forging or extruding techniques which generally limits thechemicalcomposition and sets up undesirable grain patterns in thefinished product.

It is a prime object of this invention therefore to provide an improvedalloy steel having chemical properties ideally suited for casting, toprovide manufactured parts having inherent resistance characteristicswith respect to high temperatures and products of combustion resultingfrom the operation of an internal combustion engine.

A still further-object is to provide an improved material particularlysuited to the manufacture and use of an improved cast valve.

Another object is to provide an improved stainless steel having achemical combination which particularly meets, to a maximum degree, allof the desirable characteristics of a valve element as expressed above.

A morespecific object is to provide an improved alloy steel particularlysuited to parts subjected to the conditions and products of combustionof an engine, the mate rial having among other things a silicon contentwhich particularly suits the same to the manufacture of cast parts, suchas exhaust valves, etc.

A further object is to provide a steel alloy material particularlysuited to cast parts subjected to high temperatures and corrosion, thematerial having favorable work hardening characteristics which improvethe inherently favorable characteristic of the cast article during use.

In the development of the improved cast alloy steel certain resultantfactors have been taken intoconsideration. Chromium is of courseparticularly desirable in that it provides high corrosion resistance. Upto chromium may be particularly beneficial in this respect but over thisfigure brittleness generally results. Applicants steel thereforeutilizes a chromium range of approximately 20 to 22% which assures highcorrosive resistance without the possible undesirable characteristicswhich additional chromium might produce. Carbon of course will increasethe hardness of an austenitic structure but it also may, when unitedwith chromium increase the susceptibility to corrosion. This resultswhen the carbon at higher temperatures becomes unstable and is rejectedfrom solid solutions in the form of chromium-rich carbide. Thus adjacentareas of the metal become impoverished in chromium with resultantincrease in corrosion. In the present material a range of .90% to 1.20%carbon has been found to be particularly effective.

The important characteristics of hot hardness and hot strength are notfound in predominantly chromium-ironcarbon alloys and in additiontherefore an austenizing element is necessary where the aforesaidproperties are of pronounced importance. Nickel is a well known elementfor this purpose. Nickel may however have an adverse effect on corrosionresistance while operating in the presence of lead compounds of amountsrequired for improved engine operation. Furthermore nickel is of coursealso a very scarce product. By supplanting a substantial quantity ofnickel ordinarily required for austenitic quality, with manganese, anaustenitic steel is obtained and the adverse effect of nickel uponcorrosion resistance in the combustion products of leaded fuels islargely dispelled. Accordingly in my alloy steel the nickel content isreduced to the smallest percentage 1.00% to 2%.

1 Manganese has been substituted for larger proportions of nickel. Itcontributes markedly to strength and hardness and tends to decrease thecritical cooling rate of the material. Further with increasing carbonlower ductility results. Manganese being highly resistant to sulphur andlead bearing atmospheres is also very desirable. In my alloy I utilize amanganese content of 5.00% to 8.00%. Above this figure a possibilityexists of delta ferrite formation and resultant brittleness.

Nitrogen contributes greatly to hot strength and toughness in mymaterial as well as serving to retain the austenitic structure under thecombined action of thermal and mechanical forces. This material incombination with manganese substitutes for my decrease in the nickelcontent. It further restricts grain growth while main taining itsimportant characteristic of being an austenizing element; The chromiumand manganese of my material increase the solubility of the nitrogenwhich exists in the range of 20% to 0.40%.

Wrought steel alloys normally contain less than 50% silicon. This isparticularly true of a conventional valve material where the siliconcontent is held below .25% With a low silicon content of this type theadvantages of a cast material cannot be obtained. High silicon contentis important in promoting castability of my alloy. Too low a siliconcontent will also result in poor weldability and poor scalingresistance. Too high a silicon content may cause serious brittleness. Inour cast valve material I have adapted a range of .60% to 1.20% siliconto obtain with the high carbon, high hot hardnessand a high degree ofscale resistance without adversely aifecting the corrosion resistance.Furthermore the high silicon also provided better castability of thematerial.

and age hardening. The valve in the engine is repeatedly heated andcooled which to a great extent no doubt transforms the structure of thematerial in the same way that results from age hardening. In addition,however,

that these changes area result of a combination of work;

The steel then consists of the following: certain structural change takeplace in the valve which Percent also are believed to definitely resultfrom the phenom- Carbon 0.90 to 1.20 enon of work hardening. Not onlydoes the valve de- Silicon 0.60 to 1.20 'velop greater hardness but alsothe mechanical proper- Manganese 5.00 to 8.00 ties'are definitelyimproved and it is felt that this is due Ch i 15,00 to 25,00 1 to thesteady impounding and the state of stress under Ni k l 00 to which thevalve works in the engine. In other Words Ni 0 20 to 4 this materialconsistently improves during the operation Iron Balance of the enginewhereas other valve materials known in the art deteriorate duringoperation. High hardness in- Some of the properties of this material inthe as cast 15 creases the amount of elastic deformation that can becondition are as follows: tolerated. The combination of high hardnesswith low Tensile Properties Room 1,000 F. 1,200 F. 1,400 F., 1,500? F.1,600 F.

Temp.

Ultimate Strength, p.s.l 106,650 76,150 66,850 52,400 44,300 33,335Yield Strength (2% p.s.i. 86,500 56,100 50,000 44,900 37, 500 27,850Elongation, Percentin4D. 4.29 5. 72 5.72 4.75 4.6 6.4 Reduction of Area,perceut 3. 39 5. 61 6. 12 5. 82 7. 21 6.73

Hardness after test At Room Temperature Hardness Room 1,000 F. 1,200 F.1,400 F. 1,500 F. 1,600 F.

Temp.

Brinell (750 Kg. 5 mm. Ball), At dilierent temperatures; 289 212 173 159182 89 Hardness after test At Room Temperature As indicated in my statedobjects my improved matemodulus will result in superior wear resistanceof the rial is particularly adapted for casting and thus grain 35material. Indetermining the best suited elastic modulus structureadvantages can be achieved which are not posand the higher possiblehardness, my material composisible in forged or extruded (wrought)materials. A tion was chosen to have low atomic weight sum and highwrought valve for instance stretches and distorts in meltingtemperature. In this respect my' casting matethe rim portion which leadstoburning or it becomes rial shows a great difference when compared withthe brittle and breaks under the high temperature condition. 40commercially available valve materials (wrought mate- It is believedthat this undesirable condition, in addition rials). to conventionalmaterials used, results from the grain Another important feature'ofthismaterial is'its work structure obtained incident to the forging orextruding hardening ability. Hardness of the cast material is notOperation diminished due to elevated operating temperatures but Grainsin cast material are very coarse by wrought in contradiction, it isincreased-due to work-hardening. material standards. Coarse grain size,however, is bene- Thus a valve made from my material may go into anficial since most failures are intergranular, and coarse engine at30Rockwell C? and during operation in the grained materials, having lessgrain boundary, are less engine the Rockwell hardness increases from 30to 41-45 prone to integranular failure. This is of particular RC.importance in my valve material since this material is Thus, it isreadily apparent that the'objects of the inoperating at high temperatureat which the grain vention have been fully achieved; Changes, andmodifiboundaries generally become weaker and failure more cations in thecomposition may be made which'do not likely will result. Furthermore ithas become apparent depart from the spirit of the invention, or from thescope that stress orientation in the crystal structure of a cast of theappended claims. 7 valve is considerably more favorable to theconditions What is claimed is: to which it is subjected than in a forgedvalve. The 1. A stainless steel having high resistance to hightempresent material therefore is particularly suited for castperaturesandproducts of internal combustion, containing ing. While it isespecially adapted for exhaust valves about 0.90% to 1.20% carbon, about0.60% to 1.00% it must be realized that it has particular adaptabilityfor silicon, 5% to 8% manganese, 20% to 22% chromium, any parts whichmust operate at high temperature and 1% to 2% nickel, about 0.20% to0.40% nitrogen, and under conditions where hot corrosive gases areincident the remainder substantially iron. p to the operation. 2. Astainless steel valve having'high resistance to high Another desirablefeature of my material is its hardentemperatures and products ofinternal combustion, conability. It is capable .of Work and agehardening to over taining about 0.90% to l.20% carbon, at least 0.60% to40 Rockwell C. On quenching from 2150 F. it has a 1.00% silicon, 5% to8% manganese, 20% to 25% chrohardness of about 30 Rockwell C and onreheating to mium, 1% to 2% nickel, about 0.20% to 0.40% nitrogen, 1400F. and holding it there for 8 hours, its hardness and the remaindersubstantially iron. increases to 42 to 45 Rockwell C hardness. 3. Astainlesssteel having high resistance to high tem- The terms above,namely, work and age hardening peratures and products of internalcombustion, containhave particular significance in the present valvesteel. ing about 0.90% to 1.20% carbon,'atleast.0.60% to The cast valveherein described is placed into theengine 1.20% silicon, 5% to 8%manganese, 20% to 22% chroin the as-cast condition without hardening byprecipitamium, 1% to 2% nickel, about 0.20% to 0.40% nitrogen, tion orby other means. During the use in the engine and the remaindersubstantially iron. significant changes taken place in valve and it isfelt 4. A stainless steel material particularly adapted for castinginternal combustionengine valves which during use are subjected to hightemperatures and corrosive atmospheres containing about 0.90% to.1.20%carbon, 0.60% to 1.20% silicon, about 5% to 8% manganese, 20% to 25%chromium, 1% to 2% nickel, 0.20% to 0.40% nitrogen, and the remaindersubstantially iron.

5. A stainless steel material particularly adapted for casting internalcombustion engine valves which during use are subjected to hightemperatures and corrosive atmospheres containing about 0.90% to 1.20%carbon, 0.60% to 1.20% silicon, about 5% to 8% manganese, 15% to 22%chromium, 1% to 2% nickel, 0.20% to 0.40% nitrogen, and the remaindersubstantially iron.

6. A stainless steel material particularly adapted for casting partswhich during use are subjected to high temperatures and corrosiveatmospheres containing about 0.90% to 1.60% carbon, not less than 0.60%and not more than 1.20% silicon, about 5% to 8% manganese, 20% to 25chromium, not less than 1% and not exceeding 2% nickel, 0.20% to 0.40%nitrogen, and the remainder substantially iron.

7. A stainless steel material for producing cast internal combustionengine valves containing not less than 0.90% and not more than 1.20%carbon, about 0.60% to 1.00% silicon, 5% to 8% manganese, 15% to 25%chromium, 1% to 2% nickel, 0.20% to 0.40% nitrogen, and the remaindersubstantially iron.

8. An austenitic stainless steel consisting essentially of about 0.90%to 1.50% carbon, 0.60% to 1.20% silicon, 5% to 8% manganese, 15% to 25%chromium, 1% t 2% nickel, 0.20% to 0.40% nitrogen and the remaindersubstantially all iron.

9. An austenitic alloy consisting essentially of about 0.9% to 1.0%carbon, 0.6% to 1.0% silicon, to 8% manganese, 15% to 25% chromium, 1%to 2% nickel, 0.2% to 0.4% nitrogen and the remainder substantiallyiron.

I 10. An austenitic iron-base valve having high strength and hardnessand good resistance to corrosion in the presence of leaded fuelcombustion products at valve operating temperatures and containingas'essential alloying elements carbon from about 0.9% to about 1.0%,

manganese from about 5.0% to about 8.0% silicon from about 0.6% to about1.20%, chromium from about 15.0% to about 25.0%, nickel from about 1.0%to about 2.0%, nitrogen from about 0.2% to about 0.4%, the balance beingiron with incidental impurities.

References Cited in the file of this patent UNITED STATES PATENTS

1. A STAINLESS STEEL HAVING HIGH RESISTANCE TO HIGH TEMPERATURES ANDPRODUCTS OF INTERNAL COMBUSTION, CONTAINING ABOUT 0.90% TO 1.20% CARBON,ABOUT 0.60% TO 1.00% SILICON, 5% TO 8% MANGANESE, 20% TO 22% CHROMIUM,1% TO 2% NICKEL, ABOUT 0.20% TO 0.40% NITROGEN, AND THE REMAINDERSUBSTANTIALLY IRON.